Desulfurization wastewater produced by a limestone-gypsum method in a thermal power plant contains high concentrations of Cl−, SO42−, Na+, Ca2+ and Mg2+, and is typical high-salt and high-hardness wastewater. At present, most desulfurization wastewater zero discharge projects of domestic and foreign coal-fired power plants use a process route of softening and evaporative crystallization. In order to reduce the evaporation amount, some projects adopt membrane concentration after pretreatment to reduce evaporation load. When the above zero discharge projects are in operation, a large amount of sludge and mixed salt are produced, this part of by-products have complex components and low utility value and can only be treated as solid waste. Therefore, existing technologies only achieve zero liquid discharge and do not achieve real zero discharge.
A part of the sludge produced in a system is formed by precipitating suspended matters carried by influent water, the other part of the sludge is composed of magnesium hydroxide and calcium carbonate precipitated in the dosing and softening process, the current disposal method is that the mixed sludge is subjected to pressure filtration and landfill, and due to the fact that the suspended matter content and hardness of the desulfurization wastewater are high, the amount of the generated sludge is 80-120 kg/ton of wastewater, and the landfill cost is about 48-72 RMB/ton of wastewater.
The main components of the mixed salt produced in the system are sodium chloride and sodium sulfate, and the mixed salt cannot be consumed by the market and can only be taken as solid waste for landfill. The amount of the mixed waste salt produced from evaporative crystallization of the desulfurization wastewater is 25-50 kg/ton of wastewater, and the landfill cost is about 50-100 RMB/ton of wastewater. At the same time, due to the very strong solubility of inorganic salts, landfill also has certain risks.
It can be seen that the current zero discharge technology of the desulfurization wastewater only solves the problem of wastewater discharge. The large amount of solid waste generated by the zero discharge system can only be treated by landfill. In this way, not only is the operating cost of the system significantly increased, but also resources are not used rationally, the real zero discharge is not achieved, and the original intention of the zero discharge projects is not met.